A flexible printed wiring or circuit board (hereinafter referred to as FPC) is employed in a compact electronic apparatus such as a smart phone and a tablet PC due to the easiness of wiring and the lightness. Due to the recent improvement of functionality of electronic apparatuses, the signal transmission rate has been accelerated, so that impedance matching is an important factor even for an FPC. In order to achieve impedance matching for the increased signal capacity, a resin insulating layer (e.g., polyimide) as the base of an FPC has been thickened. In order to meet the demand for densification of wirings, multilayering of an FPC has been further developed. On the other hand, when an FPC is processed for bonding to a liquid crystal substrate and mounting an IC chip, alignment is performed with a positioning pattern which is visually recognized through a resin insulating layer remained after etching of the copper foil of a laminate composed of the copper foil and the resin insulating layer. The visibility of the resin insulating layer is therefore important.
A copper clad laminate composed of a laminate of a copper foil and a resin insulating layer may be manufactured from a rolled copper foil having a roughened plated surface. The rolled copper foil is usually manufactured from tough pitch copper (oxygen content: 100 to 500 ppm by weight) or oxygen-free copper (oxygen content: 10 ppm by weight or less) as a raw material ingot, which is hot rolled and then subjected to repeated cold rolling and annealing to a predetermined thickness.
Examples of the techniques include the followings. Patent Literature 1 discloses an invention of a copper clad laminate of a polyimide film and a low profile copper foil, which allows a film after etching of the copper foil to have a light transmittance of 40% or more at a wavelength of 600 nm, with a haze value (HAZE) of 30% or less and an adhesive strength of 500 N/m or more.
Patent Literature 2 discloses an invention of a chip on flexible (COF) flexible printed wiring board having an insulating layer on which a conductive layer of electrolytic copper foil is laminated, allowing the insulating layer in an etched region after circuit formation by etching of the conductive layer to have a light transmittance of 50% or more. The electrolytic copper foil includes a rustproof layer of nickel-zinc alloy at the joint area bonded to the insulating layer. The joint area has a surface roughness (Rz) of 0.05 to 1.5 μm and a specular gloss of 250 or more at an incident angle of 60°.
Patent Literature 3 discloses an invention of a method for processing a copper foil for a printed circuit, including forming a cobalt-nickel alloy plated layer after surface roughening treatment of the copper foil surface by plating with a copper-cobalt-nickel alloy, and further forming a zinc-nickel alloy plated layer.